Such rotational dobbies have become well-known both in technical literature and also in reality, wherein for the coupling, different radially movable and controlled driving keys which can engage a groove of the drive shaft have been provided, for example as disclosed in U.S. Pat. No. 3 180 366, German Pat. No. 1 535 258 and German Offenlegungsschrift No. 30 01 310, and on the other hand pawls have been provided which are supported on the eccentric ring, are controlled in any manner and engage a groove of the drive shaft. Examples of such a driving coupling are for example illustrated and described in Swiss Pat. No. 473 253, German Offenlegungsschrift No. 29 09 131 and French Pat. No. 1 201 358.
In all of the aforementioned conventional rotational dobbies, the drive shaft rotates intermittently in successive steps of 180.degree.. It is common to effect the control of the coupling during the phase when the drive shaft stands still, wherein such standstill phase in the case of weaving machines which run with a high speed is relatively short or is limited in time. There thus resulted during the development of high-speed rotational dobbies the problem of carrying out the entire control during this limited time or constructing the coupling so that the effective duration for the control, including the reading of the pattern card and also the giving of impulses and safe adjusting of the coupling members, is expanded without limiting the working speed of the connected weaving machines.
It has proven to be disadvantageous to have control mechanisms in which the important controlling function is carried out by a spring, because an instability factor with respect to a constant timely control sequence develops and such mechanism are unreliable when viewed over a long term, which is not the case in mechanical control mechanisms which do not have a spring. During the initial tensioning of springs in anticipatorily controlled arrangements, a considerable reserve force must be produced by the control apparatus, which requires a stronger type of construction of the control apparatus. Finally, it is decisive that the thickness of the coupling elements depends on the width defined by the heddle-frame separation.
Important for the coupling operation is furthermore the safe and certain engagement of the pawls in the groove of the drive shaft. The members for the engaging operation are advantageously tapered at their ends. It is thereby disadvantageous that, during a great load on a wedge or pawl, for example upon a quick shaft start during the intermittent shaft operation, the tapered ends facilitate a pressing of the coupling parts out of the shaft groove. For example, German Offenlegungsschrift No. 30 01 310 shows a locking system which, within the given conditions, is designed short pitched and for this reason does not assure the demanded functional safety.
A further condition which is requested from a modern machine is a universally usable coupling, that is a coupling which operates both during forward and also during reverse operation of the machine. The possibility of turning the shaft or the weaving back for pick finding must be assured in dobbies.
A goal of the invention is thus a driving coupling for rotational dobbies which works in both directions of rotation, which can be anticipatorily controlled without the help of a spring, which effects a play-free coupling, and which has in the coupled condition an automatic self-block. In addition, this coupling is to be able to be installed within the heddle-frame spacing of a weaving machine and is to work error-free at a high speed.